Polyolefin resins, such as polypropylene, are being increasingly used in automotive parts and the like because of their high-performance and cost-saving benefits. However, unlike polar synthetic resins, such as polyurethane, polyamide, acrylic or polyester resins, polyolefin resins are non-polar and also crystalline, and thus cause difficulties in coating or bonding applications.
A conventional approach to solving the above problem involves activating the surfaces of polyolefin resin molded articles by plasma or gas flame treatment to thereby promote the adhesive properties of the molded articles. However, this approach has drawbacks, such as the necessity of complicated processes, which imposes a huge burden on facility costs and time, as well as possible variations in surface treatment effect owing to the shape complexity of the molded articles or the influence of pigments or other additives.
Various methods as approaches to applying polyolefin resins without such surface treatments (pretreatment) have been proposed which utilize a primer composition, such as that used in the coating of car bumpers made of polypropylene (e.g., Japanese Examined Patent Publication No. 1994-002771).
Such a primer composition generally contains, as a main component, a chlorinated polyolefin resin formed by modifying a polyolefin with an unsaturated carboxylic acid and/or an acid anhydride thereof and then chlorinating the modified polyolefin (e.g., Japanese Examined Patent Publication No. 1989-016414). Japanese Examined Patent Publication No. 1991-060872 discloses that, by graft-copolymerizing a radical polymerizable unsaturated compound with chlorinated polypropylene and liquid rubber, a binder is obtained that adheres to polyolefin resins as well as other polar resins.
However, these chlorinated polyolefin resin compositions are less compatible with resins contained in primer compositions or topcoat materials such as acrylic resins, polyester resins or polyurethane resins, and therefore cause difficulties in forming a uniform film. In addition, modified products using liquid rubber may produce a tacky coating film, thus failing to sufficiently improve compatibility and solution stability.